/**
 * @license
 * Copyright 2021 Google LLC
 * SPDX-License-Identifier: BSD-3-Clause
 */
import { ContextType, Context } from './create-context.js';
declare global {
    interface HTMLElementEventMap {
        /**
         * A 'context-request' event can be emitted by any element which desires
         * a context value to be injected by an external provider.
         */
        'context-request': ContextRequestEvent<Context<unknown, unknown>>;
    }
}
/**
 * A callback which is provided by a context requester and is called with the value satisfying the request.
 * This callback can be called multiple times by context providers as the requested value is changed.
 */
export type ContextCallback<ValueType> = (value: ValueType, unsubscribe?: () => void) => void;
/**
 * Interface definition for a ContextRequest
 */
export interface ContextRequest<C extends Context<unknown, unknown>> {
    readonly context: C;
    readonly contextTarget: Element;
    readonly callback: ContextCallback<ContextType<C>>;
    readonly subscribe?: boolean;
}
/**
 * An event fired by a context requester to signal it desires a specified context with the given key.
 *
 * A provider should inspect the `context` property of the event to determine if it has a value that can
 * satisfy the request, calling the `callback` with the requested value if so.
 *
 * If the requested context event contains a truthy `subscribe` value, then a provider can call the callback
 * multiple times if the value is changed, if this is the case the provider should pass an `unsubscribe`
 * method to the callback which consumers can invoke to indicate they no longer wish to receive these updates.
 *
 * If no `subscribe` value is present in the event, then the provider can assume that this is a 'one time'
 * request for the context and can therefore not track the consumer.
 */
export declare class ContextRequestEvent<C extends Context<unknown, unknown>> extends Event implements ContextRequest<C> {
    readonly context: C;
    readonly contextTarget: Element;
    readonly callback: ContextCallback<ContextType<C>>;
    readonly subscribe?: boolean;
    /**
     *
     * @param context the context key to request
     * @param contextTarget the original context target of the requester
     * @param callback the callback that should be invoked when the context with the specified key is available
     * @param subscribe when, true indicates we want to subscribe to future updates
     */
    constructor(context: C, contextTarget: Element, callback: ContextCallback<ContextType<C>>, subscribe?: boolean);
}
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